1. Field of the Invention
The present invention relates to a balun transformer for converting balanced signals into an unbalanced signal or vice versa, and more particularly to a 3-line balun transformer which has a simple structure and is easy to design and manufacture.
2. Description of the Related Art
Generally, “balun” is an abbreviation for ‘balance to unbalance’, which typically signifies a circuit or structure for converting balanced signals into an unbalanced signal or vice versa.
For example, a balun transformer may be required in a wireless communication field to connect a mixer, amplifier, etc. including balanced lines with components including unbalanced lines.
A balun transformer may be implemented with a combination of transmission lines or a concentrated constant circuit, or in the form of a resonant waveguide in the case where it is employed in an antenna field.
FIG. 1 is an equivalent circuit diagram of a conventional balun transformer proposed by Marchand. As shown in this drawing, the conventional balun transformer comprises four transmission lines 11˜14, each having a length of λ/4 (here, λ is 1/fc (fc is a center frequency of an input/output signal)). The first and third lines 11 and 13, and the second and fourth lines 12 and 14 form couplers, respectively. The first line 11 has its one end connected to an unbalanced port 15 for input or output of an unbalanced signal of a predetermined frequency and its other end connected to one end of the second line 12, the other end of which remains open. The third and fourth lines 13 and 14, coupled respectively with the first and second lines 11 and 12, have their one ends connected to ground and their other ends connected respectively to balanced ports 16 and 17 for input or output of two balanced signals.
In the above structure, if a signal of a predetermined frequency is applied to the unbalanced port 15, then an inter-line electromagnetic coupling occurs, thereby causing the balanced ports 16 and 17 to output signals which are the same in level and 180 degrees out of phase with each other, respectively.
To the contrary, if signals with the same levels and a phase difference of 180° therebetween are applied respectively to the balanced ports 16 and 17, then an unbalanced signal is outputted from the unbalanced port 15.
FIG. 2 is an equivalent circuit diagram of another conventional balun transformer. As shown in this drawing, the conventional balun transformer comprises first to fourth lines 21˜24 which form two couplers, in a similar manner to that of FIG. 1. The structure of FIG. 2 is different from that of FIG. 1 in that the third line 23 has its one end connected to an unbalanced port 27 and its other end connected to ground, the first line 21 and second line 22 have their one ends connected to each other and their other ends connected respectively to balanced ports 25 and 26, and the fourth line 24 has its both ends connected to the ground. If the two couplers have the same structures, they must be in symmetrical relation to each other.
However, because the balun transformers shown in FIGS. 1 and 2 are implemented with the four lines, each having the length of λ/4, there is a need for a simpler structure. Further, the balun transformer of FIG. 2 is very hard to manufacture because the two couplers thereof are proposed to have a symmetrical structure.
In order to overcome the above problems, there has been proposed a balun transformer having a simpler structure consisting of three lines, as shown in FIG. 3. This balun transformer has a structure employing an equivalent, second line 32 to replace the right coupler in the structure of FIG. 2. Here, the left coupler has a symmetrical structure.
FIG. 4 shows the structure of another balun transformer consisting of three lines. As shown in this drawing, the balun transformer comprises first to third lines 41˜43 which are arranged in parallel to form inter-line couplings. The first and second lines 41 and 42 have their one ends connected in common to an unbalanced port 44, the first line 41 has its other end connected to a balanced port 45, and the third line 43 has its one end connected to a balanced port 46. The middle or second line 42 and third line 43 have their other ends connected to ground.
The above-mentioned balun transformer 40 is simpler in structure than 4-line balun transformers, but is disadvantageous in that a branching point 44a must be formed in the unbalanced port 44, resulting in unnecessary reflection of high frequency signals.